Method for transmitting control and training symbols in multi-user wireless communication system

ABSTRACT

The present invention relates to a method and an apparatus for transmitting control and training symbols to improve transmission efficiency in a multi-user wireless communication system. The method for transmitting the control and training symbols in the multi-user wireless communication system according to one embodiment of the present invention comprises the steps of: determining whether a required transmission rate of each data can be satisfied through channel estimation in each of terminals when different data are simultaneously transmitted to each of the terminals; and transmitting a data frame to each of the terminals, the data frame being composed to discriminate the control and training symbols in each of the terminals using a combination of time, frequency, and code area when the required transmission rate of each data is not satisfied according to the determined result.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/458,993, filed Apr. 27, 2012, which is a continuation ofInternational Patent Application No. PCT/KR2010/007574, filed Oct. 30,2010, which claims priority to Korean Patent Application10-2009-0104616, filed Oct. 30, 2009, and Korean Patent Application10-2010-0013612, filed Feb. 12, 2010, which are incorporated byreference in their entirety.

TECHNICAL FIELD

Exemplary embodiments of the present invention relate to a method andapparatus for transmitting control and training symbols in a multi-userwireless communication system, which can improve the transmissionefficiency thereof.

BACKGROUND ART

Recently, transmission schemes for higher transmission rates in wirelesscommunication systems are being researched and standardized. In order tohave such a high transmission rate in wireless LAN systems as well, astructure having a transmission rate of a maximum of 600 Mbps has beenstandardized, to which a MIMO system having multiple input/output inIEEE 802.11 TGn has been applied. There has been discussion in IEEE802.11 VHTSG regarding a system having a maximum transmission rate of 1Gbps at MAC SAP, and the task group of IEEE 802.11 TGac/TGad has beenestablished accordingly. In order to maintain frequency efficiency whilesatisfying such a high transmission rate, the AP and STA must supportmore streams than four, which are supported by TGn, requiring a largenumber of antennas. In STA's terms, it is difficult to support a largenumber of antennas, considering the complexity or power consumption ofthe STA. Therefore, multi-user MIMO is being considered, according towhich the AP simultaneously transmits to multiple STAs.

FIG. 1 is a timing diagram for explaining the occurrence of interferencebetween stations (STAs) which simultaneously transmit data in a case inwhich a transmission scheme such as TGn is maintained while supporting amulti-user MIMO.

As illustrated in FIG. 1, when different data are simultaneouslytransmitted to two or more STAs, different information transmitted tothe respective STAs may be interfered in areas indicated by referencenumeral 101.

In addition, the respective STAS has different signal to interferenceplus noise ratios (SINRs), depending on channel states or interferencedegrees of the STAs. However, in a currently considered frame structure,the number of LTFs is determined by the number of streams, and a MCS isdetermined by a minimum transfer rate of a signal field (SIG).

In the IEEE 802.11n, a mixed PPDU format provides a backwardcompatibility with the IEEE. 802.11a/g, and a green field formatsupports only the IEEE 802.11n. Each STA sets Network Allocation Vector((NAV):(TXOP)) information by using length information and a modulation& coding scheme contained in a signal field of a frame.

However, in a case in which a multi-user MIMO is applied, each STAreceives a beamformed frame, and thus, STAs may not correctly detectlength information and MCS of the signal field. Consequently, a hiddennode problem may become more serious.

DISCLOSURE Technical Problem

An embodiment of the present invention is directed to an apparatus andmethod for solving a hidden node problem in a wireless communicationsystem using a multi-user MIMO.

Another embodiment of the present invention is directed to an apparatusand method for solving a hidden node problem in a green-field mode, inwhich VHT-SIG is divided into a common signal field, which can bereceived by all STAs, and a dedicated signal field, which includesbeamformed STA information, and appropriate LTF and SIG structures areselected depending on channel states or interference degrees betweenSTAs.

Technical Solution

In accordance with an embodiment of the present invention, a method fortransmitting control and training symbols in a multi-user wirelesscommunication system includes: determining whether or not a requiredtransfer rate of each data is met in each station through a channelestimation, upon simultaneous transmission of different data to eachstation; and when the required transfer rate of each data is not met,configuring a data frame so that the control and training symbols aredistinguished at each station by using the combination of time,frequency and code domains, and transmitting the data frame to eachstation.

In accordance with another embodiment of the present invention, a methodfor transmitting control and training symbols in a multi-user wirelesscommunication system includes: determining whether or not a requiredtransfer rate of each data is met in each station through a channelestimation, upon simultaneous transmission of different data to eachstation; and when the required transfer rate of each data is met,configuring a data frame so that the control and training symbols areoverlapped without being distinguished at each station, and transmittingthe data frame to each station.

Advantageous Effects

The embodiments of the present invention have the following effects.

First, the STAs having a poor channel state increase (repeat) the lengthof the LTF and applies a low MCS to the VHT-SIG-D or repeats the symbolsof the VHT-SIG-D. In this way, the VHT-SIG-D detection performance canbe improved.

Second, the STAs having a good channel state transmits the VHT-SIG-D asone or more streams and uses a high MCS to reduce the number of symbolsoccupied by the VHT-SIG-D, thereby increasing the transmissionefficiency.

Third, the channel estimation performance can be improved bycoordinating the LTF between the STAs.

Fourth, a hidden node problem caused by the beamforming in thegreen-field format can be avoided.

DESCRIPTION OF DRAWINGS

FIG. 1 is a timing diagram for explaining the occurrence of interferencebetween stations (STAs) which simultaneously transmit data in a case inwhich a transmission scheme such as TGn is maintained while Supporting amulti-user MIMO.

FIG. 2 is an exemplary diagram of a PPDU format in an IEEE802.11a/g/n/VHT mixed mode in a mode “a”.

FIG. 3 is an exemplary diagram of a PPDU format in an IEEE 802.11n/VHTmixed mode in a mode “a”.

FIG. 4 is an exemplary diagram of a green-field PPDU format inaccordance with an embodiment of the present invention.

FIG. 5 is an exemplary diagram of a PPDU of a mixed mode format in amode “b” for STAs coordinating an LTF.

FIG. 6 is an exemplary diagram of a PPDU of a green-field format in amode “b” for STAs coordinating an LTF.

FIGS. 7A to 7D are exemplary diagrams for explaining a method forcoordinating an LTF in a mode b-1, a mode b-1, a mode b-2, a mode b-3,and a mode b-4 in accordance with an embodiment of the presentinvention.

FIGS. 8A to 8H are exemplary diagrams of a spread matrix for explainingan LTF coordination process in accordance with an embodiment of thepresent invention.

FIG. 9 is a flowchart for determining a PPDU format in accordance with apreferred embodiment of the present invention.

BEST MODE

Exemplary embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstrued as limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the present invention tothose skilled in the art. Throughout the disclosure, like referencenumerals refer to like parts throughout the various figures andembodiments of the present invention.

First, a multi-user MIMO transmission/reception signal can be expressedas shown:y=HWp+n  [Equation 1]

where y denotes a reception signal, H denotes a channel, W denotes aprecoding matrix of transmitting end, p denotes a training sequencevalue and n denotes a noise.

If a ZF precoding scheme which nulls interference between STAs is used,there is no interference between STAs in the ideal environment. However,if an MMSE precoding scheme is applied, interference occurs betweenSTAs.

When assuming that an AP transmits two streams and two STAs receive onestream, a transmission/reception signal of a training sequence in amulti-user MIMO is expressed as shown below.

$\begin{matrix}{y = {{{HWp} + n} = {{{\begin{bmatrix}h_{11} & h_{12} \\h_{21} & h_{22}\end{bmatrix}\begin{bmatrix}w_{11} & w_{12} \\w_{21} & w_{22}\end{bmatrix}}\begin{bmatrix}p_{1} \\p_{2}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2}\end{bmatrix}}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Channel estimations of STA 1 and STA 2 may be expressed as Equation 3below.{tilde over (h)} ₂=(h ₁₁ w ₁₂ +h ₁₂ w ₂₂)+(h ₂₁ w ₁₁ +h ₂₂ w ₂₁)p ₁ p ₂*+n ₂ p ₂*{tilde over (h)} ₁=(h ₁₁ w ₁₁ +h ₁₂ w ₂₁)+(h ₁₁ w ₁₂ +h ₁₂ w ₂₂)p ₂ p ₁*+n ₁ p ₁*  [Equation 3]

As in Equation 3 above, interference exists between the STAs, and suchinterference becomes serious with the correlation degree of channels. Inorder to such an error, a method of increasing length by repeating anLTF, a method of reducing an MCS of a SIG or increasing symbol length,or a method of coordinating an LTF and transmitting the coordinated LTFmay be used. In addition, when a channel state is superior, a method ofincreasing an MCS and reducing an overhead may be used.

However, the above-described error reducing methods increase theoverhead of the LTF occupied in the frame. Thus, in order to reduce suchan overhead, a signal field indicating whether or not the LTF isrepeated, the MCS of the SIG is reduced, and the LTF is coordinated isrequired. The coordination of the LTF is information which must be knownby all STAs coordinating the LTF. Therefore, the information should betransmitted in such a way that all STAs can receive it, not a specificbeamforming. Hence, a VHT-SIG is divided into a common control signaland a dedicated control signal.

In this embodiment, a field which transmits the common control signal ofthe VHT-SIG is defined as a VHT-SIG-C, and a field which transmits thededicated control signal of the VHT-SIG is defined as a VHT-SIG-D. Amode for STAs which do not coordinate the LTF is defined as a mode “a”,and a mode for STAs which coordinate the LTF is defined as a mode “b”.

A mode in which the AP supports not the VHT STA but 11a/g/n STAs isdefined as an 11a/g/n/VHT mixed mode, a mode which supports the IEEE802.11n is defined as an 11n/VHT mixed mode, and a mode which does notsupport the IEEE 802.11a/g/n is defined as a green-field mode. Therespective transmission frame format is called a PPDU format.Hereinafter, a transmitting method in each mode will be described.

A transmitting method in a mode “a” will be described below.

FIG. 2 is an exemplary diagram of a PPDU format in an IEEE802.11a/g/n/VHT mixed mode in a mode “a”, and FIG. 3 is an exemplarydiagram of a PPDU format in an IEEE 802.11n/VHT mixed mode in a mode“a”.

In FIGS. 2 and 3, the PPDU format has a common phase and a dedicatedphase. The common phase is defined as a phase till a VHT-SIG-C field,and the dedicated phase is defined as a phase after the VHT-SIG-C field.

In the case of FIGS. 2(a) and 3(a), VHT-SIG-C fields 211 and 311 arelocated after an HT-SIG field. In addition, in the case of FIGS. 2(b)and 3(b), VHT-SIG-C fields 221 and 321 are located after a VHT-STFfield.

In FIGS. 2(b) and 3(b), when a VHT STA receives the IEEE 802.11n frameformat, the STA does not know whether the frame is the IEEE 802.11nframe or the VHT frame, prior to detection of the VHT-SIG-C. Thus,considering that an HT-STF for automatic gain control (AGC) may belocated at a symbol position of the VHT-SIG-C, the VHT-STF symbol may betransmitted after the HT-SIG, and then, the VHT-SIG-C may betransmitted.

In the case of FIGS. 2(c) and 3(c), VHT-SIG-C fields 231 and 331 arelocated after the VHT-LTF field. In FIGS. 2(c) and 3(c), when an AGC isperformed through the VHT-STF, the VHT-LTF is transmitted after theVHT-STF in order for decoding performance of the VHT-SIG-C, and then,the VHT-SIG-C fields 231 and 331 are transmitted.

In FIG. 2(d), after an L-SIG, a VHT-SIG-C field 241 may be immediatelytransmitted, without HT-SIG. In addition, various PPDU formats may beprovided.

In the cases of FIGS. 2(a) to 2(d) and FIGS. 3(a) to 3(c), all dedicatedphases may have the VHT-SIG-D fields 212, 222, 232, 242, 312, 322 and332.

FIG. 4 is an exemplary diagram of a green-field PPDU format inaccordance with an embodiment of the present invention.

In the cases of FIGS. 4(a) and 4(b), the green-field PPDU format may bedivided into a common phase and a dedicated phase. The dedicated phasesstart after the VHT-SIG-C fields 411 and 421. Therefore, in thededicated phases, the VHT-SIG-D fields 412 and 422 are located in thededicated phases.

More specifically, as illustrated in FIG. 4(a), information of VHT-STF2and VHT-LTF1 fields and information of VHT-SIG-D and VHT-LTF2 fields aretransmitted through the VHT-SIG-C field 411 all STAs can receive. Asillustrated in FIG. 4(b), the VHT-LTF1 may be transmitted when the AGCis unnecessary after the VHT-SIG-C 421.

FIG. 5 is an exemplary diagram of a PPDU of a mixed mode format in amode “b” for STAs coordinating an LTF, and FIG. 6 is an exemplarydiagram of a PPDU of a green-field format in a mode “b” for STAscoordinating an LTF.

In FIGS. 5 and 6, VHT-SIG-C fields 511, 521, 531, 541, 551, 561, 611,621, 631 and 641 divide the PPDU into the common phase and the dedicatedphase. The dedicated phases include the VHT-SIG-D fields 512, 522, 532,542, 552, 562, 621, 622, 532 and 642). A detailed description will bedescribed below with reference to the accompanying drawings.

FIGS. 5(a), 5(b), 5(c) and 5(d) are identical to three cases of the mode“a” in FIG. 2. Coordination between the STAs may be performed by K STASwhich simultaneously transmit data, or may be performed by necessarySTAs, for example, the STAs a to b. FIG. 5(a) illustrates a case inwhich the STAS 2 to K are coordinated. That is, the VHT-SIG-D fields 522and 532 may be located at arbitrary positions between the VHT-SIG-C anda data field, and the positions may be designated by the information ofthe VHT-SIG-C. The cases of FIGS. 5(b), 5(c) and 5(d) may coordinate theSTAs in the same manner as FIG. 5(a).

A case of FIG. 6 will be described below. The cases of FIGS. 6(a) and6(b) are identical to the three cases in the mode a of FIG. 3.Coordination between the STAs may be performed by K STAS whichsimultaneously transmit data, or may be performed by necessary STAs, forexample, the STAs a to b. FIG. 6(a) illustrates a case in which the STAS2 to K are coordinated. In addition, in the case of FIG. 6(b), STAs maybe coordinated in the same manner as that of FIG. 6(a). At this time,the VHT-SIG-D fields 612, 622, 632 and 642 may be located at arbitrarypositions between the VHT-SIG-C and a data field, and the positions maybe designated by the information contained in the VHT-SIG-C of thecorresponding frame.

A control message contained in a signal field will be exemplarilydescribed below.

Information contained in the VHT-SIG1 (common control signal, VHT-SIG-C)in which all STAs receive the same information is as follows.

The VHT-SIG1 (VHT-SIG-C) contains the following information.

(1) Mode a: STA which does not perform LTF coordination

-   -   The following information is required in each STA.

a) Symbol number of VHT-LTF1, repetition or non-repetition

b) Symbol number of VHT-LTF2 (it may be contained in VHT-SIG2(VHT-SIG-D))

c) MCS of VHT-SIG2 (VHT-SIG-D)

d) Symbol number of VHT-SIG2 (VHT-SIG-D), Repetition or non-repetition

(2) Mode b: STA which performs LTF coordination

-   -   Index of STA which performs LTF coordination)    -   LTF coordination method    -   Symbol number of VHT-LTF1, Repetition or non-repetition    -   Symbol number of VHT-LTF2 (it may be contained in VHT-SIG2        (VHT-SIG-D))    -   MCS of VHT-SIG2 (VHT-SIG-D)    -   Symbol number of VHT-SIG2 (VHT-SIG-D), repetition or        non-repetition

(3) In the case of the green-field mode, a hidden node problem caused bybeamforming is avoided, and the following information is additionallycontained in order for the case of STA which does not use beamforming

-   -   MCS, length information    -   Use or non-use of VHT-STF2

(4) The following information is contained in VHT-SIG2 (VHT-SIG-D) inwhich STAs receive different information.

-   -   Information for data area of STA, such as MCS, bandwidth (BW),        length, aggregation, short guide interval (short GI)    -   The structure of VHT-LTF2 among information contained in        VHT-SIG1 may be contained in VHT-SIG2.

LTF coordination methods may be provided depending on time-domain,frequency-domain, and code-domain coordination.

-   -   Mode b-1: Time-domain coordination    -   Mode b-2: Frequency-domain coordination    -   Mode b-3: Time-domain, code-domain coordination    -   Mode b-4: Code-domain, frequency-domain coordination

FIGS. 7A to 7D are exemplary diagrams for explaining an LTF coordinationmethod in the cases of mode b-1, mode b-2, mode b-3, and mode b-4.

In FIGS. 7A to 7D, data are simultaneously transmitted to four STAs, andeach STA receives one stream. FIG. 7A illustrates an example in whichSTAs are configured to transmit data in division by using differentsymbols which are time-domain values, and FIG. 7B illustrates an examplein which STAs are configured to transmit data in division by usingdifferent subcarriers which are frequency-domain values. FIG. 7Cillustrates an example in which STAs transmit data in division by usingsymbols, which are time- and frequency-domain values, and differentcodes in each STAs as symbol axes, and FIG. 7D illustrates an example inwhich STAs transmit data in division by using subcarriers, which arefrequency- and code-domain values, and different codes in each STAs assubcarrier axes.

In FIGS. 7A to 7D, in the cases in which the respective STAs receive nstreams, LTF corresponding to each STA is expanded to n LTFs, and theyare coordinated in each STA. Thus, the configuration can be easilyderived from one stream. Except for the combination of the mode b-1 tothe mode b-4, a new LTF coordination method can be configured fromcombinations of these modes, and such a configuration can be easilyderived from the existing modes.

In the case of LTF coordination, a transmission signal S can beexpressed as shown:

$\begin{matrix}{\begin{bmatrix}s_{11} & s_{12} & \ldots & s_{1n} \\s_{21} & s_{22} & \ldots & s_{2n} \\\vdots & \vdots & \ddots & \vdots \\s_{m\; 1} & s_{m\; 2} & \ldots & s_{mn}\end{bmatrix} = {\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1m} \\w_{21} & w_{22} & \ldots & w_{2m} \\\vdots & \vdots & \ddots & \vdots \\w_{g\; 1} & w_{g\; 2} & \ldots & w_{gm}\end{bmatrix}{\quad{\begin{bmatrix}c_{11} & c_{12} & \ldots & c_{1n} \\c_{21} & c_{22} & \ldots & c_{2n} \\\vdots & \vdots & \ddots & \vdots \\c_{m\; 1} & c_{m\; 2} & \ldots & c_{mn}\end{bmatrix}\begin{bmatrix}p_{1} & 0 & \ldots & 0 \\0 & p_{2} & \ldots & 0 \\\vdots & \vdots & \ddots & \vdots \\0 & 0 & \ldots & p_{n}\end{bmatrix}}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$where p is an LTF sequence, and n is a symbol index corresponding tosymbol area. C is a code which spreads into time or frequency ortime/code or time/frequency domain, constituting a spread matrix. As thespread matrix, an orthogonal matrix, a discrete Fourier transform (DFT)matrix, and a unitary matrix may be used. m is an spatial time streamindex corresponding to a spatial domain, and is equal to a total sum ofthe number of spatial time streams when STAs intending to simultaneouslytransmit data to MU-MIMO are coordinated. w is a precoding matrix fortransmission of MU-MIMO, and g is a TX antenna index.

The case of OFDM can expand to a subcarrier which is the unit of thefrequency domain, and a subcarrier index is skipped in Equation 4 above.

For example, when the mode is spread to the time domain like in the modeb-1 and the time-domain unit is a symbol, only the diagonal elements ofthe spread matrix has values, off-diagonal elements are zero. This isillustrated in FIG. 8A.

FIG. 8A is an exemplary diagram of a spread matrix when the mode b-1 isspread to the time domain and the time-domain unit is a symbol. In FIG.8A, a horizontal axis is a symbol, and a vertical axis is a spatial timestream.

In addition, for example, when the mode is spread to the frequencydomain like the mode b-2 and the frequency-domain unit is a subcarrier,only the diagonal elements of the spread matrix have values, like theexpansion of the time domain. This is illustrated in FIG. 8B.

FIG. 8B is an exemplary diagram of a spread matrix when the mode b-2 isspread to the frequency domain and the frequency-domain unit is thesubcarrier. In FIG. 8B, a horizontal axis is a subcarrier, and thevertical axis is a spatial time stream.

In addition, for example, the spread matrix may be illustrated like inFIG. 8C, when the mode is spread to the time and code domains like themode b-3 and the time-domain unit is a symbol.

FIG. 8C is an exemplary diagram of a spread matrix when the mode b-3 isspread to the time and code domains and the time-domain unit is asymbol. In FIG. 8C, a horizontal axis is a symbol, and a vertical axisis a spatial time stream.

In addition, for example, when the mode is spread to the frequency andcode domains like the mode b-4 and the frequency-domain unit is asubcarrier, the spread matrix may be illustrated like FIG. 8D.

FIG. 8D is an exemplary diagram of a spread matrix when the mode b-4 isspread to the frequency and code domains and the frequency-domain unitis a subcarrier. In FIG. 8D, a horizontal axis is a subcarrier, and avertical axis is a spatial time stream.

By combining the above-described schemes, the spread matrix can beconfigured by easily expanding in the symbol/subcarrier form in whichthe symbol and the subcarrier are combined. When assumed that the totalspatial time stream to be transmitted is allocated in each STA, it maybe exemplified like FIG. 8E.

FIG. 8E is an exemplary diagram when the total spatial time stream to betransmitted is allocated in each STA.

Referring to FIG. 8E, STA 1 uses two spatial time streams, STA 2 usesthree spatial time streams, and STA K uses one spatial time stream. Asillustrated in FIG. 8E, all STAs need not use the same number of thespatial time streams.

For example, when the total six spatial streams are used by three STAs,that is, each STAs uses two spatial time streams, and the spread matrixuses a DFT matrix, the allocation of the spread matrix in each STA maybe illustrated like FIG. 8F.

In FIG. 8F, a horizontal axis is a spatial time stream, and a verticalaxis is a symbol, a subcarrier, or a symbol/subcarrier. In the spreadmatrix of FIG. 8F, the values of the first row and the first columnare 1. In addition, it should be noted that they have a value ofx=exp(−j2π/6).

In addition, for example, when the total eight spatial streams are usedby four STAs, that is, each STAs uses two spatial time streams, and aunitary matrix having real values is used as the spread matrix, theallocation of the spread matrix in each STA may be illustrated like FIG.8G.

In FIG. 8G, a horizontal axis is a spatial time stream, and a verticalaxis is a symbol, a subcarrier, or a symbol/subcarrier. As illustratedin FIG. 8G, each element value of the spread matrix may have anarbitrary value. As described above, the spread matrix may be a DFTmatrix or a unitary matrix.

When the number of the spatial time streams to be simultaneouslytransmitted to the MU-MIMO is four and two STAs transmit two spatialtime streams, respectively, the symbols required in the time domain isfour. Thus, the calculation of the spread matrix can be performed as inFIG. 8H by applying 4×4 partial matrix which is a part of 8×8 matrix.

FIG. 9 is a flowchart for determining a PPDU format in accordance with apreferred embodiment of the present invention.

At step 900, the AP collects channel information of each STA throughsounding or feedback information. At step 902, interference between theSTAs is estimated from the channel information collected at step 900 byapplying a precoding algorithm, such as ZF, MMSE, Sphere encoder, and soon.

At step 904, after the interference estimation, the AP determineswhether or not the STAs meet necessary performance. This step is donefor distinguish STAs which do not meet the required performance becausea channel estimation error is increased by an increased interferencebetween the STAs. That is, the STAs which do not meet the requiredperformance perform an LTF coordination, and the STAs which meet therequired performance do not perform an LTF coordination.

When the determination result of step 904 is met, that is, when theVHT-LTF coordination is not performed, the AP operates in a mode “a”. Inthis case, the AP proceeds to step 906 to determine MCS of VHT-SIG-D byusing the estimated SINR of the STA. When the estimated SINR is high,higher MCS is applied to the VHT-SIG-D, instead of BPSK. When theestimated SINR is low, the lowest MCS is transmitted.

On the other hand, when the determination result of step 904 is not met,that is, when the VHT-LTF coordination is performed, the AP operates ina mode “b”. In this case, the AP proceeds to step 908 to select anappropriate coordination mode by using mobility, delay spread, SINRinformation of STAs which are coordinated by the AP.

For example, the AP applies the mode b-3 when the delay spread is largeand applies the mode b-4 when the delay spread is small. When the SINRis low and the delay spread is large, the AP reduces the number of thesimultaneous transmission users and applies the mode b-3 to obtain again by a dispreading.

The AP proceeds to step 910 to determine whether or not theVHT-LTF/VHT-SIG is repeated, and determine the number of repetition ofthe VHT-LTF/VHT-SIG. That is, when the AP coordinates the LTF, it canrepeat the LTF in order to further improve the channel estimationperformance. Thus, the number of repetition of the VHT-LTF/VHT-SIG isdetermined. In addition, the AP can increase the detection probabilityof the dedicated control signal by repeating the VHT-SIG-D.

As described above, when the mode and repetition for transmission aredetermined at steps 906 and 910, the AP proceeds to step 912 todetermine a PPDU format, and configures the PPDU and transmits theconfigured PPDU.

In the mode a described above with reference to FIG. 2, the receivingend operates as follows in the 11a/g/n/VHT mixed mode.

First, the case of FIG. 2(a) will be described below.

1) The receiving end performs a carrier sensing, an AGC, a timingsynchronization, and a coarse frequency offset estimation through anL-STF.

2) Then, the receiving end performs a fine frequency offset estimationand a channel estimation through an L-LTF.

3) Then, the receiving end decodes an L-SIG by using the channelestimation value obtained using the L-LTF.

4) Then, the receiving end detects an HT-SIG using an HT-SIG detectionmethod (BPSK phase rotation), and decodes it using the channelestimation value of the L-LTF.

5) After the above procedures, the receiving end detects a VHT-SIG-Cusing a VHT-SIG-C detection method (BPSK phase rotation), and decodes itusing the channel estimation value of the L-LTF.

6) The receiving end performs the AGC on the beamformed multi-user MIMOsignal using the VHT-STF.

7) Then, the receiving end estimates the multi-user MIMO channel throughthe VHT-LTF by using information on the VHT-LTF structure of theVHT-SIG-C.

8) Then, the receiving end decodes the VHT-SIG-D from the information onthe VHT-SIG-D indicated by the VHT-SIG-C and the channel estimationvalue using the VHT-LTF.

9) The receiving end decodes data using the information on the VHT-SIG-Ddata.

Next, the case of FIG. 2(b) will be described below. In the case of FIG.2(b), the steps 1) to 4) are identical to those of the case of FIG.2(a). Thus, only the subsequent steps will be described.

5) After the decoding of the L-SIG, the receiving end performs an AGC byusing VHT-STF.

6) Then, the receiving end detects a VHT-SIG-C using a VHT-SIG-Cdetection method (BPSK phase rotation), and decodes it using the channelestimation value of the L-LTF.

7) Then, the receiving end performs an AGC on the beamformed multi-userMIMO signal using the VHT-STF.

8) The receiving end estimates the multi-user MIMO channel through theVHT-LTF by using information on the VHT-LTF structure of the VHT-SIG-C.

9) Then, the receiving end decodes the VHT-SIG-D from the information onthe VHT-SIG-D indicated by the VHT-SIG-C and the channel estimationvalue using the VHT-LTF.

10) The receiving end decodes data using the information on theVHT-SIG-D data.

Next, the case of FIG. 2(c) will be described below. In the case of FIG.2(c), the steps 1) to 4) are identical to those of the case of FIG.2(a). Thus, only the subsequent steps will be described.

5) After the decoding of the L-SIG, the receiving end performs an AGC byusing VHT-STF.

6) Then, the receiving end performs a channel estimation using theVHT-LTF.

7) The receiving end detects a VHT-SIG-C using a VHT-SIG-C detectionmethod (BPSK phase rotation), and decodes it using the channelestimation value of the L-LTF.

8) Then, the receiving end performs an AGC on the beamformed multi-userMIMO signal using the VHT-STF.

9) Then, the receiving end estimates the multi-user MIMO channel throughthe VHT-LTF by using information on the VHT-LTF structure of theVHT-SIG-C.

10) Then, the receiving end decodes the VHT-SIG-D from the informationon the VHT-SIG-D indicated by the VHT-SIG-C and the channel estimationvalue using the VHT-LTF.

11) The receiving end decodes data using the information on theVHT-SIG-D data.

Next, the case of FIG. 2(d) will be described below. In the case of FIG.2(d), the steps 1) to 3) are identical to those of the case of FIG.2(a). Thus, only the subsequent steps will be described.

4) After the decoding of the L-SIG, the receiving end detects aVHT-SIG-C using a VHT-SIG-C detection method (BPSK phase rotation), anddecodes it using the channel estimation value of the L-LTF.

5) Then, the receiving end performs an AGC on the beamformed multi-userMIMO signal using the VHT-STF.

6) Then, the receiving end estimates the multi-user MIMO channel throughthe VHT-LTF by using information on the VHT-LTF structure of theVHT-SIG-C.

7) Then, the receiving end decodes the VHT-SIG-D from the information onthe VHT-SIG-D indicated by the VHT-SIG-C and the channel estimationvalue using the VHT-LTF.

8) The receiving end decodes data using the information on the VHT-SIG-Ddata.

As described above, the receiving method in 11n/VHT mixed mode/VHTgreen-field mode in the mode a and the mixed mode and the green-fieldmode in the mode b can be easily configured from the above operationstructures.

INDUSTRIAL APPLICABILITY

The embodiments of the present invention can be applied to the cases oftransmitting a training symbol in a high-rate wireless communicationsystem.

The invention claimed is:
 1. A communication method, comprising:generating, at the AP, a first sequence; generating, at the AP, a secondsequence; generating, at the AP, a first field including firstinformation; generating, at the AP, a second field including secondinformation; generating, at the AP, a third field including thirdinformation; generating, at the AP, a fourth field including fourthinformation; generating, at the AP, first user data for a first station;generating, at the AP, second user data for a second station;sequentially transmitting, from the AP, the first sequence, the secondsequence, the first field and the second field to the first station andthe second station; and transmitting, from the AP, the third field andthe first user data to the first station, and the fourth field and thesecond user data to the second station, wherein the third informationcomprises first control information related to the first user data butnot related to the second user data, the fourth information comprisessecond control information related to the second user data but notrelated to the first user data, and wherein a part of the thirdinformation indicates length of the first user data for the firststation in a frame and a part of the fourth information indicates lengthof the second user data for the second station in the frame.
 2. Themethod of claim 1, wherein the frame sequentially comprises the firstsequence, the second sequence, the first field, the second field, thethird field and the first user data.
 3. The method of claim 1, whereinthe third field further indicates a modulation and coding scheme (MCS)for the first user data.
 4. The method of claim 2, wherein the framefurther comprises a third sequence subsequent to the second field andprior to the third field, and the third sequence is used to improveautomatic gain control estimation in a multiple input multiple output(MIMO) transmission.
 5. The method of claim 1, wherein the third fieldand the fourth field are simultaneously transmitted.
 6. An apparatus foran access point (AP), the apparatus comprising: a circuity, wherein thecircuitry is configured to: cause the AP to generate a first sequence;cause the AP to generate a second sequence; cause the AP to generate afirst field including first information; cause the AP to generate asecond field including second information; cause the AP to generate athird field including third information; cause the AP to generate afourth field including fourth information; cause the AP to generatefirst user data for a first station; cause the AP to generate seconduser data for a second station; cause the AP to sequentially transmitthe first sequence, the second sequence, the first field and the secondfield to the first station and the second station; and cause the AP totransmit the third field and the first user data to the first station,and the fourth field and the second user data to the second station,wherein the third information comprises first control informationrelated to the first user data but not related to the second user data,the fourth information comprises second control information related tothe second user data but not related to the first user data, and whereina part of the third information indicates length of the first user datafor the first station in a frame and a part of the fourth informationindicates length of the second user data for the second station in theframe.
 7. The apparatus of claim 6, wherein the frame sequentiallycomprises the first sequence, the second sequence, the first field, thesecond field, the third field and the first user data.
 8. The apparatusof claim 6, wherein the third field further indicates a modulation andcoding scheme (MCS) for the first user data.
 9. The apparatus of claim7, wherein the frame further comprises a third sequence subsequent tothe second field and prior to the third field, and the third sequence isused to improve automatic gain control estimation in a multiple inputmultiple output (MIMO) transmission.
 10. The apparatus of claim 6,wherein the third field and the fourth field are simultaneouslytransmitted.
 11. The method of claim 5, wherein the first sequence is ashort training field, the second sequence is a long training field, thethird sequence is a short training field, and the fourth sequence is along training field.
 12. The method of claim 5, wherein the firstsequence is a legacy short training field (L-STF) and the secondsequence is a legacy long training field (L-LTF), the third sequence isa non-legacy short training field, and the fourth sequence is anon-legacy long training field.
 13. The method of claim 1, wherein thefirst control information is applicable to the first user data but notapplicable to the second user data, and the second control informationis applicable to the second user data but not applicable to the firstuser data.
 14. The apparatus of claim 10, wherein the first sequence isa short training field, the second sequence is a long training field,the third sequence is a short training field, and the fourth sequence isa long training field.
 15. The apparatus of claim 10, wherein the firstsequence is a legacy short training field (L-STF) and the secondsequence is a legacy long training field (L-LTF), the third sequence isa non-legacy short training field, and the fourth sequence is anon-legacy long training field.
 16. The apparatus of claim 6, whereinthe first control information is applicable to the first user data butnot applicable to the second user data, and the second controlinformation is applicable to the second user data but not applicable tothe first user data.
 17. A communication method, comprising: detecting,at a station, a first sequence; detecting, at the station, a secondsequence; detecting, at the station, a first field including firstinformation; detecting, at the station, a second field including secondinformation; detecting, at the station, a third field including thirdinformation; and detecting, at the station, user data for the station,wherein the second information is used by the station and anotherstation, the third information is used by the station but not used bythe another station, and a part of the third information indicateslength of the user data for the station in a frame, and wherein thethird information comprises first control information related to theuser data but not related to another user data detected by the anotherstation.
 18. The method of claim 17, wherein the frame sequentiallycomprises the first sequence, the second sequence, the first field, thesecond field, the third field and the user data.
 19. The method of claim17, wherein the third field further indicates a modulation and codingscheme (MCS) for the user data.
 20. The method of claim 18, wherein theframe further comprises a fourth sequence subsequent to the thirdsequence and prior to the third field, wherein the first sequence is alegacy short training field (L-STF) and the second sequence is a legacylong training field (L-LTF), the third sequence is a non-legacy shorttraining field, and the fourth sequence is a non-legacy long trainingfield.
 21. An apparatus for a station, comprising: a circuitry, whereinthe circuitry is configured to: cause the station to detect a firstsequence; cause the station to detect a second sequence; cause thestation to detect a first field including first information; cause thestation to detect a second field including second information; and causethe station to detect a third field including third information; andcause the station to detect user data for the station, wherein thesecond information is used by the station and another station, the thirdinformation is used by the station but not used by the another station,and a part of the third information indicates length of the user datafor the station in a frame, and wherein the third information comprisesfirst control information related to the user data but not related toanother user data detected by the another station.
 22. The apparatus ofclaim 21, wherein the frame sequentially comprises the first sequence,the second sequence, the first field, the second field, the third fieldand the user data.
 23. The apparatus of claim 21, wherein the thirdfield further indicates a modulation and coding scheme (MCS) for theuser data.
 24. The apparatus of claim 22, wherein the frame furthercomprises a fourth sequence subsequent to the third sequence and priorto the third field, wherein the first sequence is a legacy shorttraining field (L-STF) and the second sequence is a legacy long trainingfield (L-LTF), the third sequence is a non-legacy short training field,and the fourth sequence is a non-legacy long training field.
 25. Acommunication apparatus, , comprising: a circuitry, wherein thecircuitry is configured to: cause the apparatus to detect a firstsequence; cause the apparatus to detect a second sequence; cause theapparatus to detect a first field including first information; cause theapparatus to detect a second field including second information; andcause the apparatus to detect a third field including third information;and cause the apparatus to detect user data for the apparatus, whereinthe second information is used by the apparatus and another apparatus,the third information is used by the apparatus but not used by theanother apparatus, and a part of the third information indicates lengthof the user data for the apparatus in a frame, and wherein the thirdinformation comprises first control information related to the user databut not related to another user data detected by the another apparatus.26. The apparatus of claim 25, wherein the frame sequentially comprisesthe first sequence, the second sequence, the first field, the secondfield, the third field and the user data.
 27. The apparatus of claim 25,wherein the third field further indicates a modulation and coding scheme(MCS) for the user data.
 28. The apparatus of claim 26, wherein theframe further comprises a fourth sequence subsequent to the thirdsequence and prior to the third field, wherein the first sequence is alegacy short training field (L-STF) and the second sequence is a legacylong training field (L-LTF), the third sequence is a non-legacy shorttraining field, and the fourth sequence is a non-legacy long trainingfield.
 29. A communication apparatus, comprising: a circuitry whereinthe circuitry is configured to: cause the apparatus to generate a firstsequence; cause the apparatus to generate a second sequence; cause theapparatus to generate a first field including first information; causethe apparatus to generate a second field including second information;cause the apparatus to generate a third field including thirdinformation; cause the apparatus to generate a fourth field includingfourth information; cause the apparatus to generate first user data fora first station; cause the apparatus to generate second user data for asecond station; cause the apparatus to sequentially transmit the firstsequence, the second sequence, the first field and the second field tothe first station and the second station; and cause the apparatus totransmit the third field and the first user data to the first station,and the fourth field and the second user data to the second station,wherein the third information comprises first control informationrelated to the first user data but not related to the second user data,the fourth information comprises second control information related tothe second user data but not related to the first user data, and whereina part of the third information indicates length of the first user datafor the first station in a frame and a part of the fourth informationindicates length of the second user data for the second station in theframe.
 30. The apparatus of claim 29, wherein the frame sequentiallycomprises the first sequence, the second sequence, the first field, thesecond field, the third field and the first user data.
 31. The apparatusof claim 29, wherein the third field further indicates a modulation andcoding scheme (MCS) for the first user data.
 32. The apparatus of claim30, wherein the frame further comprises a fourth sequence subsequent tothe third sequence and prior to the third field, wherein the firstsequence is a legacy short training field (L-STF) and the secondsequence is a legacy long training field (L-LTF), the third sequence isa non-legacy short training field, and the fourth sequence is anon-legacy long training field.
 33. The apparatus of claim 29, whereinthe third field and the fourth field are simultaneously transmitted.